高质感的锌锰铁氧体

Structure and magnetic properties of highly textured nanocrystalline

Mn–Zn ferrite thin?lm

Jaison Joseph a,n,R.B.Tangsali b,V.P.Mahadevan Pillai c,R.J.Choudhary d,D.M.Phase d,

V.Ganeshan d

a Department of Physics,Goverment College,Khandola,Goa403107India

b Department of Physics,Goa University,Taleigao Plateau,Goa403206India

c Department of Optoelectronics,University of Kerala,Thiruvananthapuram,Kerala695581India

d UGC-DAE-CSR Indore,Madhya Pradesh452017India.

a r t i c l e i n f o

Article history:

Received23June2014

Received in revised form

8September2014

Accepted10September2014

Available online19September2014

Keywords:

Nanocrystalline

Thin?lms

Texture

Magnetic properties

a b s t r a c t

Nanoparticles of Mn0.2Zn0.8Fe2O4were chemically synthesized by co-precipitating the metal ions in

aqueous solutions in a suitable alkaline medium.The identi?ed XRD peaks con?rm single phase spinal

formation.The nanoparticle size authentication is carried out from XRD data using Debye Scherrer

equation.Thin?lm fabricated from this nanomaterial by pulse laser deposition technique on quartz

substrate was characterized using XRD and Raman spectroscopic techniques.XRD results revealed the

formation of high degree of texture in the?lm.AFM analysis con?rms nanogranular morphology and

preferred directional growth.A high deposition pressure and the use of a laser plume con?ned to a small

area for transportation of the target species created certain level of porosity in the deposited thin?lm.

Magnetic property measurement of this highly textured nanocrystalline Mn–Zn ferrite thin?lm revealed

enhancement in properties,which are explained on the basis of texture and surface features originated

from?lm growth mechanism.

1.Introduction

The growth of soft magnetic ferrite thin?lms on amorphous

substrates?nd scienti?c interest due to its potential commercial

applications in magnetic thin?lm read heads,micro-inductors and

micro-transformers[1–6].This interest also stems from the fact

that the magnetic properties in thin?lms show discrepancies such

as lower saturation magnetization and higher coercivity in com-

parison to its corresponding bulk materials.Ferrite?lms also?nd

applications in microwave acoustic and short wavelength magne-

tostatic wave devices,wherein the current bulk ferrite compo-

nents are not compatible with planar circuit design[7].Relatively

high resistivity and permeability of spinel ferrites in thin?lm form

make it a preferred material for soft magnetic under layers in

perpendicular magnetic recording media[8].

Many industrial applications require ferrite thin?lms to be

grown with processes compatible with integrated circuit technol-

ogy on amorphous substrates.To avoid unwanted chemical reac-

tions with the metal atoms involved in device structures during

the integration of magnetic oxides with semiconducting materials,

low processing temperature is a preferred prerequisite.An

increased mobility for the deposited atoms on the?lm surface

results in creation of homogeneous?lm microstructure,maintain-

ing the stoichiometry of the target.The high kinetic energy of the

vaporized materials in laser plume,offers pulse laser deposition

(PLD)the processing advantages favorable for above requirements.

It has been further suggested that this technique may possibly

lower the substrate processing temperature required for crystal-

lization[9]

Manganese zinc ferrite(Mn–Zn ferrite)is one prominent soft

magnetic material known for its high permeability,large resistiv-

ity,relatively high magnetization and low coercivity.It?nds

applications in magnetic sensors,reading heads for magnetic

recording media,switch mode power supplies,de?ection yoke

rings and spintronic devices[10,11–14].High quality Mn–Zn ferrite

thin?lms?nd process applications for the miniaturization of

complex geometrical devices.For example,in the arena of mag-

netic recording,a possible thermally assisted magnetic recording

method can overcome the thermal agitation limit of recording

media to achieve a higher recording density beyond Tb/inch2by

applying the magnetic recording?eld and the laser beam to the

same position on the recording media[15].Incorporating a

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journal homepage:https://www.360docs.net/doc/ac14095491.html,/locate/physb

Physica B

https://www.360docs.net/doc/ac14095491.html,/10.1016/j.physb.2014.09.015

0921-4526/&2014Elsevier B.V.All rights

reserved.

n Corresponding author.Tel./fax:t918322287718.

E-mail address:jaisonjosephp@https://www.360docs.net/doc/ac14095491.html,(J.Joseph).

Physica B456(2015)293–297

magnetic core in the recording head for a suf?ciently large recording?eld,could possibly interfere with the optical path of the laser beam.Mn–Zn ferrite thin?lm which is a transparent magnetic material with a high M s and high resistivity could possibly be a potential candidate for use as a magnetic core that will not interrupt the laser beam[16].

Based on thin?lm preparation conditions such as substrate temperature,pressure,growth rate,shadowing effects etc.and depending on the?lm growth methods one can obtain signi?-cantly different surface morphologies and?lm microstructures [17].In an event of non-commencement of epitaxial growth of a ?lm in the layer-by-layer mode,the growth front can be rough in the form of mounds.Further a noise induced roughening during the growth can lead to the formation of self-af?ne fractal morphologies[18].Another factor that strongly alters the growth characteristics is the kinetic effect stress relaxation in between ?lm interfaces.These inherently related dynamic growth mechan-isms have a signi?cant and generally different in?uence on the physical properties of the material in thin?lm format[19].

Since magnetic properties play an important role in industrial applications related to magnetic recordings and magnetic mem-ories,the amount of disorder at the surface in?uencing magnetic properties in thin?lms attain reasonable signi?cance.The size, shape and grain orientation de?ne the microstructure dependent property of any single phase polycrystalline material.In?lms with reduced grain sizes such as nanocrystalline ferrite thin?lms the role of grain boundaries is expected to gain signi?cant importance. In comparison to a bulk material the grain boundaries occupy a signi?cant volume in thin?lms.Therefore it is expected to observe a reasonable change in property due to altered grain boundary structures in thin?lms even though the grain size,chemical composition and phase remain the same.Hence we viewed the deposition of nanocrystalline ferrite thin?lm with a manifestation of high crystallographic texture as an interesting feature for investigation and studied its magnetic properties.In the present paper we report an enhanced coercivity.

2.Experimental

Nanoparticle Mn0.2Zn0.8Fe2O4ferrite material was prepared in our laboratory by co-precipitating aqueous solutions of ZnSO4, MnCl2and FeCl3mixtures in alkaline medium.The powdered sample of prepared material was characterized using Rigaku X-ray diffractometer(XRD)with a high intensity rotating anode X-ray source.The powdered sample was palletized at15T pressure and sintered at4501C temperature which was used as a target material for laser ablation.The laser ablation was carried out using Excimer laser KrF(248nm)(Lambda Physik COMPex201model) in a chamber maintaining the pressure at2?10à6T on a quartz substrate elevated to a temperature of4501C.The deposition was performed for30min keeping the substrate at a distance of4.5cm from target and retaining laser energy at220mJ with a repeation rate of10Hz.The focused laser beam was incident on the target surface at an angle of451.The target was rotated at10rpm with the substrate mounted opposite to the target on a heater plate using silver paint.After deposition,the thin?lm was cooled down to room temperature at a rate of51C/min,maintaining vacuum in deposition chamber.The prepared thin?lm was characterized using standardθ/2θXRD using Bruker AXE D8X-ray Diffract-ometer with Cu Kαradiation at room temperature.Raman spectro-scopic measurements of thin?lm was obtained on HORIBA Jobin Yvon LabRAM HR800Micro Raman spectrometer with Argon Iron Laser source having wavelength514nm in a spectral resolution of 1cmà1.The thickness measurement of the?lm was carried out on an AMBIOS XP-1stylus pro?ler with0.5nm resolution.The AFM image of the?lm taken on SPM(Digital Nano-Scope-III)in contact mode was used for?lm surface analysis and particle size determi-nation.Magnetic measurements were carried out on Quantum Design's MPMS SQUID VSM.

3.Results and discussion

The X-ray diffraction pattern of Mn0.2Zn0.8Fe2O4target material in powder form along with that of deposited thin?lm on quartz substrate is shown in Fig.1.

The positions of the observed peaks are in agreement with the peaks reported in JCPDS?les which con?rm the single phase cubic spinel structure of the target sample.The interplanar spacing(d)is calculated in accordance with Bragg's law and hence the average lattice parameter(a)is obtained using the equation given below: 1

hkl

???????????????????????

h2tk2tl2

e1TThe particle sizes of the sample was determined by the Debye Scherrer formula given below by averaging the overall seven major spinnel peaks.

0:9λ

βCosθ

e2TThe lattice constant and particle size is tabulated in Table1.

From the observation of a single XRD peak(311),the crystal structure of the thin?lm appears to be highly textured.In PLD process,strain may be induced during the?lm growth,wherein the ablation temperature is lower than the melting temperature of the target material.In such cases the?lm growth is far from equilibrium and the ablated material particles do not have enough mobility to achieve the lowest thermal dynamic energy state. During the deposition of the thin?lm a large strain could be induced in the interface between the?lm and the quartz substrate at the initial stage of growth.Further,the difference of thermal coef?cient between the?lm and quartz substrate may also induce strain/stress during?lm growth.Since the?lm was deposited at optimized temperature and deposition rate,the formation of311 texture is attributed to the minimization of strain energy density [20]

Fig.1.XRD pattern of Mn0.2Zn0.8Fe2O4.

Table1

Lattice constant and particle size of target material.

Lattice constant,a(?)Particle size,D nm

8.47482944.00

J.Joseph et al./Physica B456(2015)293–297 294

It is observed that there is an appreciable shift in peak position (blue shift)for thin ?lm 311peak in comparison to the target material.This peak shift arising out of swing towards large inter-reticular distance can be explained with the help of atomic peening phenomena [21].The deposition of the target species on substrate using PLD involves disintegration of the species from the target by high energy pulsed laser,transportation of the same through the laser plume and its deposition onto the substrate preceded by a bombardment.A bombardment on the substrate layer by energetic species generates a compressive stress in crystal planes parallel to the ?lm surface,which in turn generates an expansion in the planes normal to the surface.This results in an increase of the lattice parameter normal to the surface,which explains the shift observed in thin ?lm [22,23].Pictographic representation of the suggested phenomenon in Fig.2provides a visual representation of the happening wherein a 0,represent lattice parameter of the unstressed structure,a ┴lattice parameter normal to the surface,and a //lattice parameter parallel to the surface.

Thickness of the prepared thin ?lm determined on stylus prolifreometer and the average grain size of target material grown on substrate,derived from AFM data is as tabulated in Table 2

The formation of spinel phase in thin ?lm,which could not be judged entirely using the single peak XRD data owing to highly textured nature of the ?lm,was con ?rmed by Raman spectro-scopic technique.The Raman spectra of deposited thin ?lm are shown in Fig.3

Spinel ferrites in crystalline form arrange itself in a cubic structure that belongs to the space group O h 7(Fd3m ).Although a full unit cell occupy 56atoms (Z ?8),the smallest Bravais cell in crystal lattice consists only of 14atoms (Z ?2).As a result,the factor group analysis predicts the following modes in a spinel structure [24]:

A1g (R)tE g (R)tF1g t3F 2g (R)t2A 2u t2E u t4F 1u (IR)t2F 2u

The emission from ?ve ?rst-order Raman active modes (A 1g tE g t3F 2g ),were observed in Raman spectra of thin ?lm sample.In cubic spinel ferrites,the modes at above 600cm à1is expected to match up with motion of oxygen in tetrahedral AO4groups [25].So the prominent mode at 607cm à1can be reason-ably considered as A 1g symmetry.The other four low frequency modes represent the characteristics of octahedral sites (BO6).Therefore the observation of ?ve Raman emission peaks

originating from ?ve ?rst-order Raman active modes (A 1g tE g t3F 2g )con ?rm formation of spinal phase in thin ?lm.

The particles size and surface morphology of the ?lm were observed using AFM images (Figs.4and 5).AFM analysis of the thin ?lm showed homogeneous morphology surface with sphe-rical particles'of regular size and uniform distribution.The particles diameters on the thin ?lm surface were measured at different points by a particle analysis tool and the average size calculated was 47nm which is comparable to the size of the nanoparticles in the target material.The roughness histogram of the AFM image is as shown in Fig.6.The high level of roughness is attributed to the formation of a ridge like morphology which is predominantly visible in the AFM image of the sample.

The reason for ridge like morphology observed in the AFM image is explained in a twin phenomenon approach.The target material which is highly stable is ablated using PLD,transported in the plume and deposited onto the substrate.The substrate being quartz an amorphous material,the expected nucleation sites for the growth of the ?lm appear to be the ?rst arrived target species which are deposited onto the substrate.The unit species

Fig.2.Effect of the stress on the lattice parameter.

Table 2

Thickness and particle size of thin ?lm.Thickness (?)Particle size D (nm)1422

Fig.3.Raman spectra of Mn 0.2Zn 0.8Fe 2O 4thin ?

lm.

Fig.4.AFM images of Mn 0.2Zn 0.8Fe 2O 4thin ?lm.

J.Joseph et al./Physica B 456(2015)293–297295

reaching the substrate plane lose their velocity component normal to the substrate and are physisorbed (weakly bound)onto the surface.The adsorbed species are not in equilibrium with each other and migrate on the surface (2-d gas)until they interact with other adsorbed species and form clusters.These clusters continue to grow until they reach a critical radius where they are thermo-dynamically stable which form the nuclei.The number of nuclea-tion sites depends on adsorption properties of the target material,substrate,its combination and the physical parameters such as ambient pressure,substrate temperature,incoming species energy etc.Less nucleation sites provide an opportunity for directional growth which is optimized by a suitable selection of physical environmental parameters during growth which resulted in crea-tion of the highly textured ?lm.

A high deposition pressure and the use of Laser plume con ?ned to a small area for transportation of the target species make it possible to obtain a certain level of porosity in deposited ?lms.This can be explained by the fact that the ?ow of particles extracted from the target is concentrated onto a small volume (due to the con ?nement of particles in plume),which increases the number of particle collisions occurring in the space between the target and the substrate resulting in an increase of mean

incident angle θ2(see Fig.7).This can lead to the generation of shadowing effects leading to creation of intergranular voids within a growing layer.This along with the formation of ridge like morphology due to the directional growth described earlier con-tributes to the high level of roughnes on the surface of the ?lm.Magnetic properties of ferrite thin ?lms depend on the magnetic-crystalline anisotropy,the grain size and surface mor-phology of the ?lms.The M –H loop of the thin ?lm recorded at room temperature with sample in-plane with magnetic ?eld is shown in Fig.8.

The substrate contribution to the M –H loop was subtracted after obtaining the loop.It can be seen that the material is magnetically ordered in the thin ?lm format at room temperature and fully saturates at higher applied magnetic ?elds.The X and Y -axis intercept values indicative of coercivity and remnant magne-tization of thin ?lm sample with reasonably low hysteresis loss factor is tabulated in Table 3.

It is observed that our fabricated ferrite material in thin ?lm format poses a relatively high value of coercivity in comparison to the reported value of 140Oe [26].A substantial shift in the XRD peak position for thin ?lm sample indicating the presence of relatively large lattice strain,along with morphology disturbance noticed in AFM data explained in principle by taking into account the directional ?lm growth and generation of shadowing effect leading to creation of intergranular voids,may be the

major

Fig.5.3D AFM image of Mn 0.2Zn 0.8Fe 2O 4thin ?

lm.

Fig.6.Roughness histogram of Mn 0.2Zn 0.8Fe 2O 4thin ?lm.Horizontal scale:-.height (nm)and vertical scale:-number of

events.

Fig.7.Shadowing effect leading to creation of intergranular

voids.

Fig.8.M –H loops of Mn 0.2Zn 0.8Fe 2O 4thin ?lm.

Table 3

Coercivity and remnant magnetization of thin ?lm sample.

X -axis intercept Y -axis intercept 179.53 6.01?10à5à173.94

à5.82?10à5

J.Joseph et al./Physica B 456(2015)293–297

296

contribution in creation of large magnetic anisotropy which eventually lead to the observation of high coercivity[21–23,26,27].

4.Conclusion

Highly textured Mn0.2Zn0.8Fe2O4thin?lm was grown on quartz substrate using polycrystalline ferrite nanoparticle material as the target by PLD technique.The?lm was characterized using XRD and Raman spectroscopic techniques.The blue shift observed for311 XRD peak,arising out of swing towards large inter-reticular distance,is explained with the help of atomic peening phenom-ena.Film particle size estimation from AFM data was corroborated with the size of nanoparticles in the target material.Morphology disturbance noticed in AFM data is caused by directional?lm growth and porosity generated from shadowing effects leading to creation of intergranular voids.The M–H loop of thin?lm recorded at room temperature revealed an enhancement in coercivity which is attributable to texture enhanced lattice strain and surface disorder originated from thin?lm growth mechanism.Thus,the present work can provide an effective approach to grow high quality ferrite thin?lms in processes compatible with integrated circuit technology on amorphous substrates wherein high value of coercivity?nd potential industrial applications. Acknowledgments

The?rst author acknowledges the use of PLD,XRD,GIXRD, AFM,and SQUID VSM from UGC-DAE-CSR Indore.

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